1907.] XATUHAli SCIK.\('i:>^ OF PIIir.ADELPIIIA. 529 



additional evidence which points to this as the true iuterprotation. 

 Although the name "macroinere" is evidently a misnomer in this 

 case, it nevertheless seems well to retain the name for these small, 

 cells. Wilson (98, p. 21) calls attention to the fact that these macro- 

 meres "are relatively not very much smaller than in some of the 

 mollusks," for example Plo7iorhis (Rabl, SO). Judging from the 

 figures, however, the relative size of the cells varies considerably in 

 different species of polyclads. In Planocern inquUina these cells are 

 relatively very small as compared to the fourth ([uartct. 



Hallez (79) first interpreted these small cells at the oral pole (macro- 

 meres) as mesoderm. Selenka in his earlier work called them pharyn- 

 geal cells, and in his later papers primitive endoderm cells. Goette 

 (82) and Lang (84) designate them as lower endoderm. and believe that 

 they give rise to part of the alimentary canal. These four cells form 

 one of the chief landmarks np until a very late stage of segmentation. 

 At a time when the ectoderm is well established in a layer, and just 

 before it begins to invaginate at the lower pole to form the pharynx, 

 one can still make out these four cells. I"p until this time they have 

 not divided. Their later history is exceedingly difficult to follow. I 

 am inclined to the view that these cells do not take part in the forma- 

 tion of any structures in the embryo of Planocera, but that they degen- 

 erate and that their substance is absorbed by the endoderm cells. The 

 reasons for this view are, first, that they cannot be traced to any organ, 

 and, secondly, that the nuclei of these cells from the time of their 

 formation until the last trace that can be found of them show increasing 

 evidence of degeneration. Soon after these cells are formed it can be 

 seen that the chromatin is massed on one side of the nucleus in a dark 

 staining mass (figs. 17 and 20, PI. XXXVII, and fig. 25, PI. XXXVII, 

 etc.). It is this marked evidence of degenerating nuclei that enables 

 one to follow these cells even until the ectoderm has reached. this pole 

 of the egg. In the other nuclei of the egg the chromatin is more or less- 

 evenly distributed in granules often along distinct linin threads. We 

 thus have the remarkable and unique phenomena of the "maa-omeres"' 

 deqeneratiwj icithoid giving rise to any part of the embryo. Evidently 

 the function of the macromeres in this case has been taken over by the 

 cells of the fourth ({uartet. These latter cells, as the figures show, are 

 by far the largest cells in the embryo and contain the great bulk of the 

 food yolk. In fact three of these cells, 4a, 4b and 4c, probably function 

 entirely as the bearers of yolk, and if they do take part in the formation 

 of the alimentary canal it is only a minor part. The history of the 

 other member of the fourth quartet, 4d, is of especial interest and will 

 35 



